Programming overview

Transcription

1 Programming overview

2 Basic Java A Java program consists of: One or more classes A class contains one or more methods A method contains program statements Each class in a separate file MyClass defined in MyClass.java Compiled to java bytecode To define MyClass.class

3 Basic Java // comments about the class public class MyProgram {.,88 0,/07.,88-4/ } ,3-05,.0/, 2489,3 070

4 Basic Java // comments about the class public class MyProgram { // comments about the method public static void main (String[] args) { } 209 4/-4/ 209 4/ 0,/07 }

5 Hello World // HelloWorld.java public class HelloWorld { public static void main (String[] args) { System.out.println( Hello World! ); } }

6 Program Structure Typical Java program consists of User written classes Java Application Programming Interface (API) classes Java application Has one class with a main method Java program basic elements: Packages Classes Data fields Methods

7 Packages Provide a mechanism for grouping related classes package statement Indicates a class is part of a package Java assumes all classes in a particular package are contained in same directory Java API consists of many predefined packages

8 Packages import statement Allows you to use classes contained in other packages Package java.lang is implicitly imported to all Java code

9 Figure 1-11 A simple Java Program

10 Classes An object in Java is an instance of a class Class definition includes Optional subclassing modifier Optional access modifier Keyword class Optional extends clause Optional implements clause Class body

11 Inheritance A class can inherit the members of another class. A class that is derived from another class is called a subclass (also a derived class, extended class, or child class). The class from which the subclass is derived is called a superclass (also a base class or a parent class). Excepting Object which is a system defined class and has no superclass, every class has one and only one direct superclass (single inheritance). In the absence of any other explicit superclass, every class is implicitly a subclass of Object.

12 Class hierarchy

13 Inheritance A subclass inherits all the public and protected members (data, methods, and nested classes) from its superclass. Constructors are not members, so they are not inherited by subclasses, but the constructor of the superclass can be invoked from the constructor of a subclass using the super keyword. A subclass does not inherit the private members of its parent class. However, if the superclass has public or protected methods for accessing its private fields, these can also be used by the subclass.

14 Inheritance example

15 What You Can Do in a Subclass The inherited fields (data member) can be used directly, just like any other fields. You can declare a field in the subclass with the same name as the one in the superclass, thus hiding it (not recommended). You can declare new fields in the subclass that are not in the superclass. The inherited methods can be used directly as they are. You can write a new instance method in the subclass that has the same signature as the one in the superclass, thus overriding it. You can declare new methods in the subclass that are not in the superclass. You can write a subclass constructor that invokes the constructor of the superclass, either implicitly or by using the keyword super.

16 Nested Classes The Java programming language allows you to define a class within another class. Such a class is called a nested class. class OuterClass {... class NestedClass {... } } A nested class is a member of its enclosing class and, as such, has access to other members of the enclosing class, even if they are declared private.

17 Why Use Nested Classes? It is a way of logically grouping classes that are only used in one place. It increases encapsulation. Nested classes can lead to more readable and maintainable code.

18 Classes definition components Figure 1-21 Components of a class

19 Data Fields Class members that are either variables or constants Data field declarations can contain Access modifiers Use modifiers Modules

20 Data Fields Figure 1-31 Modifiers used in data field declarations

21 Methods Used to implement operations Should perform one well-defined task Method modifiers Access modifiers and use modifiers Valued method Returns a value Body must contain return expression;

22 Method Modifiers Figure 1-41 Modifiers used in a method declaration

23 How to Access Members of an Object new operator Creates an object or instance of a class The newly created object has brand new members (data and methods) except for the static members. The static members are common between all the objects of a class. Data fields and methods declared public Name the object, followed by a period, followed by member name Members declared static Use the class or object name, followed by a period, followed by member name

24 Interface. In its most common form, an interface is a group of related methods with empty bodies. To implement this interface, you need to define a class that implements all the methods in the interface class.

25 Variables Represents a memory location Contains a value of primitive (for instance int) type or a reference. (it cannot be both) Primitive types are types where the name of the variable evaluates to the value stored in the variable. Reference types in Java are types where the name of the variable evaluates to the address of the location in memory where the object is stored.

26 Primitive Data Types Figure 1-51 Primitive data types and corresponding wrapper classes

27 References Public class aclass{ aclass(int value){m=value; } public int m; } aclass o = new aclass(225); o 225 The variable o is a reference to the object

28 References aclass o = new aclass(225); o = null; o Makes o not refer to the object any more 225

29 References aclass o = new aclass(225); o = null; o Makes o not refer to the object any more 225 The object gets deleted by Java s automated garbage collection

30 References aclass o = new aclass(225); aclass q = o; o 225 q This makes another reference to the object --- but no new object is created!!

31 References aclass o = new aclass(225); aclass q = o; q.m=0; o q

32 References aclass o = new aclass(225); aclass q = new aclass(125); o=q; o 225 The object gets deleted by Java s automated garbage collection 125 q

33 Parameter passing in Java - by reference or by value? int x = 0; givemeaten (x); System.out.println (x); [...] void givemeaten (int y) { y = 10; } Output: 0 aclass x = new aclass(0); givemeaten (x); System.out.println (x.m); [...] void givemeaten (aclass y){ y.m = 10; } Output: 10 X: 0 Y:10 x y 10 Myth: "Objects are passed by reference, primitives are passed by value"

34 aclass x = new aclass(0); aclass y = new aclass(10); swap (x, y); System.out.println (x.m+y.m); [...] void swap (aclass p, aclass q){ aclass temp=p; p = q; q = temp; } x 0 temp p Output: y q

35 Parameter passing in Java-The truth Truth #1: Everything in Java is passed by value. Truth #2: The values of variables are always primitives or references, never objects.

36 Arrays in Java To declare an array follow the type with (empty) []s int[] grade; //or int grade[]; //both declare an int array In Java arrays are objects so must be created with the new keyword To create an array of ten integers: int[] grade = new int[10]; Note that the array size has to be specified, although it can be specified with a variable at run-time

37 Arrays in Java When the array is created memory is reserved for its contents Initialization lists can be used to specify the initial values of an array, in which case the new operator is not used int[] grade = {87, 93, 35}; //array of 3 ints To find the length of an array use its.length variable int numgrades = grade.length; //note: not.length()!!